1. Field of the Invention
The invention concerns hearing aids and their assembly and is especially concerned with the long-felt need to avoid the amplification of noise caused by vibrations of either the casing or the components of the hearing aid.
2. Description of the Related Art
Hearing aids, particularly in-the-ear and in-the-canal aids, have become exceeedingly small. The casing of such a hearing aid usually contains both a microphone and a loud speaker (usually called a "receiver") which, because of their tiny size, are both delicate and difficult to handle. Their close proximity in the casing makes it difficult to avoid acoustic feedback. The microphone can additionally pick up and amplify noise from vibrations in the casing such as can be caused by external sources such as the wearer's footsteps.
The delicate nature of the receiver and microphone makes them subject to damage from shock such as when the hearing aid is accidentally dropped, as often happens because of the tiny size of the hearing aid and because its external surface often is slippery. The tiny size and tapered shape of an in-the-canal hearing aid makes it susceptible to come loose and fall from the wearer's ear.
In order to make them easier to handle and less susceptible to damage, each of the receiver and microphone are often fitted into a tiny rubber boot. For example, see U.S. Pat. No. 3,448,224 (Giller). See also the discussion of prior art in U.S. Pat. No. 4,620,605 (Gore et al.) where the boot is called a "buffer" or a "rubber bucket." The boot that the Gore patent calls "prior art" has radially extending rubber spikes which serve to locate each of the boots within a rigid plastic frame. Boots take up valuable space, and when they have spikes, they take up even more space, thus interfering with the trend toward miniaturization that is so important in current hearing aid design.
In the invention of the Gore patent, the ends of each boot are formed to permit it to be suspended in air between two fixed points and thus isolated as much as possible from structure-borne vibrations. Air suspension tends to require even more space than a rubber boot.
After the receiver and microphone have been inserted into the casing of a hearing aid, a potting compound is sometimes poured into the casing, but this makes it impractical to recover any of the parts. U.S. Pat. No. 4,520,236 (Gauthier), which concerns packing an acoustic foam material around the receiver, says that this "substantially prevents mechanical vibrations of the receiver from being transmitted to the earmold, thereby preventing feedback from this source" (col. 3, lines 22-30).
In U.S. Pat. No. 4,617,429 (Bellaflore), each of the receiver and microphone is housed in a nondescript, sleeve-like member into which a quick setting silicone material is poured. "The silicone material as used to fix the components in place also acts as a insulating medium to insure greater fidelity of sound received in the auditory canal of the user" (col. 5, lines 44-47).
In U.S. Pat. No. 4,729,451 (Brander et al.), a shaped mandrel is placed inside the casing of a hearing aid and the space between the mandrel and the casing is filled with a polymerizable liquid such as a room temperature vulcanizing silicone. After removing the mandrel, a receiver is inserted into the cavity created by the mandrel and thus is cradled by the polymerized silicone. This is said to lower the level of mechanical and acoustic feedback transmitted by the receiver.
In addition to the above-discussed techniques that have been used in attempts to reduce noise amplification, some hearing aids include electronic devices to filter out noise. Not only are electronic devices quite expensive, but they also can take up valuable space.